The sulfur cycles are a group of thermochemical processes that can make hydrogen, mainly using high temperature thermal energy from a high temperature heat source. The Westinghouse Sulfur Process (WSP; also known as the HyS or Hybrid Sulfur Process), FIG. 1 and the Sulfur Iodine (S/I) Process, FIG. 2 are two processes in this category. The high temperature heat sources are any that produce heat, available for use, above about 800° C., such as a High-Temperature Gas-Cooled Nuclear Reactor (HTGR) or a natural gas fired combustor.
The Westinghouse Sulfur Process produces hydrogen in a low-temperature electrochemical step, in which sulfuric acid and hydrogen are produced from sulfurous acid. This reaction can be run at between 0.17 and 0.6 volts with a current density of about 500 ma/sq.cm at about 60° C. The second step in the cycle is the high temperature decomposition of sulfuric acid at 760° C. or above. Previous work by Westinghouse has identified catalysts and process designs to carry out this reaction in concert with an HTGR such as the PBMR. The final step in this process is absorption of the SO2 in water at room temperature to form sulfurous acid and a SO2 free stream of O2. This is a well known process which is hereby defined as “standard WSP”:H2SO4⇄SO3+H2O⇄SO2+O.5O2+H2O (>760° C. heat required);   (1)SO2+2H2O+0.5O2⇄H2SO3+H2O+O.5O2 (T<100° C.); and  (2)H2O+H2SO3→H2+H2SO4 (electrolyzer at about 100° C. or less).  (3)
The Iodine/Sulfur Process also starts with a reversible reaction where sulfuric acid is decomposed at over 760° C. to form sulfur dioxide as above, followed by the reaction of the sulfur dioxide with Iodine to form HI. This is a well known process which is hereby defined as “standard S/I”:H2SO4⇄SO3+H2O⇄SO2+O.5O2+H2O (>760° C. heat required);   (1)I2+SO2+2H2O+O.5O2+excess H2O⇄2HI+H2SO4+O.5O2+excess H2O (about 100° C. to 200° C. heat generated); and  (2)2HI⇄H2+I2 (greater than 400° C. heat required).  (3)
The common step in both processes is:H2SO4⇄SO2+H2O+0.502 
These standard WSP and standard S/I processes are described in detail by Lahoda et al. in U.S. Publication No. US 2006/0002845 A1.
Goosen et al.“Improvements in the Westinghouse Process for Hydrogen Production” American Nuclear Society Global Paper #88017, American Nuclear Society Annual Winter Meeting New Orleans, Louisiana, USA, November 2003, also describes the Westinghouse Sulfur Process and compares it with the Sulfur Iodine Process. Westinghouse process economics are described as well as integration with a nuclear High Temperature Gas Cooled Reactor (HTGR) such as a Pebble Bed Modular Reactor (PBMR) to drive reactions by delivering temperatures of about 800° C.-900° C. to H2SO4 reactors in the WSP or S/I Processes.
After the H2SO4 Decomposition Reactor (a process step common to both the standard WPS and S/I processes), the hot gas from the primary loop of a HTGR is sent to a Power Conversion Unit (PCU), consisting of a set of gas turbines where the remaining energy is extracted. Unfortunately, after extraction of the high quality heat needed for H2SO4 decomposition, the temperature of the gas is no longer high enough to take full advantage of the high efficiencies available from preferred Brayton cycle gas turbines.
The portion of the process where sulfuric acid is decomposed into sulfur dioxide, water vapor and oxygen, 12 in FIG. 1, typically takes place at high temperatures. Due to the lower condensation temperature of the Decomposition Reactor product stream compared to the feed stream, the SO2/H2O/O2 outlet of the preheater/vaporization unit, 20 in FIG. 1, must be kept at a relatively high temperature, typically approximately 260° C. at 9 MPa. (1 MPa=145 pounds/sq. in “psi”). As a result, the amount of heat that can be recovered in the preheater is limited. The SO2/O2/H2O stream must eventually be cooled to approximately 90° C. before being introduced to SO2 scrubbers, shown generally as 14 in FIG. 1, so that this cooling duty represents a significant loss of energy.
The preheating/vaporization unit, 20 in FIG. 1 presents a severe materials issue as well. As it is evaporated, water is boiled off first, so that this stream changes in the Preheater from a relatively dilute H2SO4 solution of around 30% to 50% by weight, to a concentrated solution of 80% to 95% by weight H2SO4 when the highest temperatures are reached. While there are materials that can operate at the required temperatures (200° to 700° C.), they are very expensive.
Another inefficiency that is built into the process is evaporation and condensation of water that enters with the feed H2SO4. The sulfuric acid feed is typically 30% to 50% weight, so that a large amount of water is preheated and evaporated, only to be condensed and recirculated. The heat of vaporization of this water represents another substantial energy penalty. The decomposition process is typically aided by the presence of catalysts, and any water in the feed to the decomposition process can significantly reduce the decomposition catalyst life.
After cooling, the SO2/O2/H2O stream is sent to the SO2 scrubber and O2 recovery until, generally shown as 14 in FIG. 1, where SO2 is absorbed into water to make sulfurous acid (H2O+SO2⇄H2SO3). This sulfurous acid is then fed to an aqueous phase electrolysis cell, 16 in FIG. 1, where the sulfurous acid is oxidized to sulfuric acid, generating hydrogen product (2H2O+SO2⇄H2+H2SO4). In order for the electrolysis step to work efficiently, the SO2 must remain dissolved in the water. Sulfur dioxide has a limited aqueous solubility. Increasing the pressure increases the amount of SO2 that can be absorbed into the scrubber solution, decreases the level of SO2 in the O2 product, and decreases the amount of makeup SO2 that must be provided. However, increasing pressure also increases the cost of the process equipment, especially the vessels that contain the electrolysis units.
Thus, there continually remains a need to reduce operating temperatures and pressures so that low cost steels can be used and to increase efficiencies. It is a main object of this invention to provide a system using low temperatures, low pressures and high efficiencies.